In a gas condensate reservoir, the dense gas contained in the reservoir has liquid hydrocarbons in varying amounts dissolved in it, depending upon the geologic conditions of deposition and upon pressure and temperature conditions in the reservoir. The gas condensate can occur at various strata within the reservoir, wherein the amount of liquid yield at each depth may vary. Therefore, it is important to be able to estimate the amount of liquid hydrocarbon contained within the gas condensate at each depth of the reservoir.
One tool used to determine conditions within a reservoir is a pulse neutron capture logging device, exemplified by the PDK-100 device manufactured by Western Atlas International, Inc. The PDK-100 instrument uses a fast neutron (14 MeV) accelerator and two scintillation gamma ray detectors to measure sigma, the macroscopic neutron absorption cross section of sub-surface formations including fluids.
The PDK-100 instrument fires short bursts of fast neutrons into the formation at a rate of one burst per millisecond. Gamma rays, induced by neutron interactions in the borehole and the formation are measured and sorted into 100 equal-width gates for each of the two detectors. Two complete decay curves are thus obtained, each representing the neutron absorption rates in both the borehole and the formation.
The pulse neutron capture logging tool responds to both inelastic and capture gamma rays. When the formation is bombarded with neutrons, the neutrons are initially traveling very fast and when a neutron bounces off another atom, that atom emits gamma rays. These gamma rays are called inelastic gamma rays. As the neutrons travel further into the formation they lose energy and eventually arrive at a low enough energy state to allow them to be captured by other atoms. As a neutron is captured by an atom, gamma rays are again emitted. These gamma rays are called capture gamma rays. These two types of gamma rays are measured by the pulse neutron capture device and the ratio of these two gamma rays is plotted in a RICS (Ratio of Inelastic to Capture Spectra) plot.
As discussed above, the pulse neutron capture device contains two gamma ray detectors, called the near and far detectors. As the captured gamma rays are received by the pulse neutron capture device, they are detected by both the near and far detectors, and the ratio of the capture gamma rays by these two detectors is plotted in a curve called RATO.
The RICS and RATO plots respond to the type of minerals and fluids in the reservoir, including the amount of liquid within a gas condensate. The primary response of the RICS and RATO plots, however, is to the minerals within the formation, and not to the amount of liquid in the gas/condensate. Because the primary response is not to the amount of liquid, in the prior art, the amount of liquid indicated by the RICS and RATO plots could only be visually estimated by viewing the amount of separation between the RICS and RATO plots, to provide a qualitative estimate.
There is need in the art for a method of evaluating the RICS and RATO plots to provide a quantitative measurement of liquid yield within a reservoir. There is a further need for such an apparatus and method that adjusts for the porosity and minerals within the formation when estimating the yield. The present invention meets this and other needs in the art.